1. Field of the Invention
This invention relates to an optical target and apparatus and method for automatic identification thereof, particularly, but not exclusively, suitable for photogrammetry measurement tasks.
2. Discussion of Prior Art
Photogrammetry systems use optical targets attached to an object of interest, and the photogrammetry system is able to determine the two dimensional and three dimensional locations of the targets in order to measure the dimensions of, or position of the object. It is often useful to assign a value or code to each of the targets so as to distinguish the targets from one another. The ability to identify specific targets has particular application in identifying specific objects, determining object orientations, matching measured points to computer aided design (CAD) data and denoting known or control target locations. These capabilities are important where photogrammetry systems are to be embedded in assembly processes and operations. Furthermore, if target identities can be determined in the two-dimensional data, the subsequent determination of three-dimensional co-ordinates is faster, more efficient, and more resistant to errors.
Apparatus to distinguish optical targets from each other are available, but these use optical targets having groupings of various size and shape retro-reflective targets, which are arranged in set geometrical position with respect to each other, positioned around a positioning sub target. Although these schemes enable adequate identification of optical targets, the processing involved can be inefficient and can thus slow down the measurement process. Furthermore, dynamic measurements of target or object velocities for fast moving objects are not practicably possible.
There is therefore a need for a generally improved optical target, together with apparatus and method for automatic identification thereof, which apparatus and method facilitate target identification from two-dimensional data by low level processing.
According to a first aspect of the present invention there is provided an optical target including a positioning sub target for determining position, and a plurality of identifying sub targets for identifying one optical target from another, each of which plurality of identifying sub targets is a different colour from the positioning sub target.
Preferably the positioning sub target is substantially circular and fabricated from retro-reflective material.
Conveniently the identifying sub targets are each substantially circular and fabricated from retro-reflective material.
Advantageously the identifying sub targets are locatable substantially equi-spaced with respect to one another and substantially co-linear with the positioning sub target.
Preferably there is provided colour filters superimposed upon said identifying sub targets to provide the various colours characterising the identifying sub targets.
Conveniently there is a first filter colour and a second filter colour.
Advantageously the first filter colour is green and the second filter colour is red.
Preferably there are one said positioning sub target and three said identifying sub targets comprising the optical target.
According to a further aspect of the present invention there is provided apparatus for automatically identifying optical targets including light source means operable to project light onto said positioning and identifying sub targets.
Preferably the lights source means includes at least a first light source and a second light source, each of which is a circular array of light emitting diodes.
Conveniently the first light source has a circular array of green light emitting diodes and wherein the second light source has a circular array of red light emitting diodes.
Advantageously there is provided switching means to continuously switch between said first and second light sources.
According to a further aspect of the present invention the apparatus includes measurement means operable to image light reflected from the positioning and identifying sub targets when light is projected thereon from the first light source means.
Preferably the measurement means includes a carrier, at least two imaging devices, each attachable to the carrier, configurable to image the optical target, and each having one of the light sources attachable thereto, a processor in operative association with the imaging devices, and communication links between the imaging devices and the processor for transmitting output signals indicative of the light reflected by the positioning and identifying sub targets.
Conveniently each of the imaging devices is a metrology sensor operable to create digitisable images.
Advantageously there is provided three optical targets.
According to a yet further aspect of the present invention there is provided a method for automatically identifying optical targets including the steps of projecting light from light source means, including at least two light sources, each of which is one of two different colours, onto at least one optical target, which optical target includes a positioning sub target and a plurality of identifying sub targets, which identifying sub targets include colours corresponding to the colours of the light sources, digitally imaging onto measurement means a distribution of light reflected from the optical target, which reflected light is representable as bright pixels on at least two imaging devices forming part of the measurement means, transmitting signals indicative of the distribution of light from the at least two imaging devices means to a processor, and processing the signals, thereby to automatically identify the optical target.
Preferably the projection of light from the light sources includes operating switching means at a desired switching rate so as to continuously switch power between said light sources.
Conveniently the processing of signals includes the steps, for each of the optical targets, of assigning each of the two light source colours a logic value so as to correspondingly represent each bright pixel imaged by the measurement means with a binary value, decoding the signals from each of the two imaging devices into a first and a second array of binary value, which first and second arrays of binary values relate to the first and second imaging devices respectively, and matching elements of the first array with those of the second array so as to identify optical targets between first and second imaging devices.